This invention relates to a focusing-error detecting apparatus usable in a system for optically reproducing, recording, or erasing information from, or on an optical or magneto-optical recording medium.
An optical recording medium such as an optical disk is formed with a pattern of a groove or pits which represents recorded information. Some optical pickup heads are used in reproducing information from such an optical recording medium. In general, the optical pickup head applies a laser light beam to the optical recording medium and detects the laser light beam reflected back from the optical recording medium. Since the reflected laser light beam depends on a pattern on the optical recording medium, the reflected laser light beam represents information on the optical recording medium. Thus, the detection of the reflected laser light beam enables the reproduction of the information.
Such optical pickup heads have three basic functions. The first function is to focus the laser light beam into a micro-spot on the optical recording medium. The size of the light beam spot preferably corresponds to a diffraction limit. The second function relates to focusing control of the light beam. The third function relates to tracking control of the light beam.
In general, focusing control includes detection of a focusing error. A spot size detection method is known as a typical method of detecting a focusing error. As will be explained later, a prior-art focusing-error detecting apparatus based on a spot size detection method has some problem.